Polynuclear Nickel(II) Complexes: A Magnetostructural Study of Ni^II₄, Ni^II₆, and Ni^II₉ Species with Oxime Ligands

Abstract

Syntheses, crystal structures, and magnetic properties are reported for a tetrametallic (complex <b>1</b>), a nonametallic (complex <b>2</b>), and two hexametallic (complexes <b>3</b> and <b>4</b>) nickel(II) clusters, namely, [Ni₄(HL¹)₂(μ-OAc)₂(MeOH)] (<b>1</b>), [Ni₉(L²)₁₀(μ₃-OH) ₂ (μ-OH) ₂ (μ-OH ₂)₂(OH₂)₆](ClO₄) ₄ (<b>2</b>), [Ni₆(L²)₉(L²H)(MeOH)(H₂O)₂](ClO₄)₃ (<b>3</b>), and [Ni₆(L³)₃(μ₃-O)₂](ClO₄)₂ (<b>4</b>), where H₄L¹ represents N,N′-dimethyl-N,N′-ethylene-bis(5-bromo-3-formyloxime-2-hydroxybenzylamine); HL², 1-methylimidazole-2-aldoxime; and H₂L³, N,N′-bis(2,3-butanedionemonoxime-2-ene)-3(aminomethyl)benzylamine. The structure of <b>1</b> can be considered as two face-sharing bioctahedral units of [Ni₂(μ-O_phen)₂(μ-OAc)] bridged by a two-atom (−N−O−) oximate linker. The Ni^II···Ni^II distances of av. 2.935 Å preclude metal−metal bonding, although they are remarkably short. Variable-temperature magnetic susceptibility data are fitted to obtain the parameters <i>J</i> ₁ = +8.0 cm⁻¹, <i>J</i>₂ = −16.0 cm ⁻¹, and <i>g</i> = 2.19 (<i>H</i>̂= −2 <i>JS</i>_<i>i</i>·<i>S</i>_<i>j</i>). The ferromagnetic coupling <i>J</i> ₁ operates between the nickel(II) centers in the face-sharing bioctahedral units, whereas <i>J</i>₂ represents the antiferromagnetic interactions mediated by a single (−N−O−) bridge separating the two nickel centers at a distance of ∼4.71 Å. A rationale for the disparate nature of interactions based on a comparison with those reported in the literature is forwarded. The structure of <b>2</b> consists of two [Ni₄(L²)₅] units linked covalently to a central nickel atom by four oximate and two hydroxy oxygen atoms, resulting in a central octahedral NiO₆ core and thus yielding the nonanuclear nickel(II) cluster. The magnetic data were analyzed by a “two-<i>J</i>” model, yielding pairwise antiferromagnetic exchange interactions, <i>J</i>₁ = −24.0 cm ⁻¹ and <i>J</i>₂ = −5.8 cm ⁻¹, between the nickel centers. The spin ground state of <i>S</i>_t = 1.0 has been confirmed by magnetization measurements (variable-temperature, variable-field) at different fields. The structure of <b>3</b> contains six nickel(II) centers, each of which is six-coordinated but with different coordination environments: NiN₆, NiO₆, NiN₃O₃(2×), NiN₄O₂, and NiNO₅. The ground-state spin has been observed to be <i>S</i>_t = 1.0 with the axial zero-field splitting parameter <i>D</i> = −7.2 cm⁻¹. Complex <b>4</b> is a rare example of dimeric [Ni₃(μ₃-O)]⁴⁺ units, in which each of six nickel(II) centers is in square-planar geometry with low-spin d⁸ Ni(II) centers, thus rendering diamagnetism to complex <b>4</b>.